The Fox .45 has always been relatively
friendly to operate having escaped the over-compression problems
that plagued so many Fox engines in later years. Over its long
production life there have been several head button variations,
some very unusual, but all seemed to function quite well. The
current button is still useable in stock form, being only slightly
over-compressed, in our opinion. Over the years, problems tended
to be mostly carburetor related, especially when the old 2-jet
carburetor was used. The introduction of the MK-X series improved
things a lot, but the current EZ series has, again, resulted
in some carburetor related problems. Check out our Carburetors
section for more information on specific carburetors and recommended
fixes.

Although the .45BB has been a
pretty good engine over the years, recently we stumbled upon
a way to improve the current version. A friend had stripped the glow plug threads
in his .45 head button and needed a replacement right away to
compete in a fun fly event that weekend. Unfortunately, we did
not have a spare button for the .45 on hand just at that moment,
and before removing one from one of our own engines to get this
fellow going, we decided to try an experiment. We did have spare
buttons for the .46ABC, and it only took a few minutes to turn
the button down on a lathe so it would fit into the slightly
smaller bore of the .45. When the engine was test run, we were
surprised to find it ran about 500rpm faster than a typical current .45.
Now curiosity had the better of us and we just had to verify
this by taking the button from one of our own .45's and and swapping
it with the modified .46 button. We swapped back and forth several
times, and the results were consistent. The engine had, indeed,
picked up at least 500rpm. Not only that, the handling qualities
improved somewhat as well in that the needle setting became broader
and less critical. So, it would seem the .45 prefers the somewhat
more open combustion chamber shape provided by the .46 button.

Looking through several decades of Fox advertising for the .45 it is interesting to note the large variation in claimed performance over the years. When it was first introduced Fox claimed 14,000rpm with a 10-6. A year later 14,500 was claimed, and by the early 1980's, an amazing 15,000rpm. Then a gradual decline again, finally ending at the current claim of 13,500rpm with a 10-6. Although we never achieved 15,000 rpm on any of our 45's, we did reach into the 14's. Our experiments with the .46 button lead us to believe that head button changes over the years are at least partly responsible for the lower rating of the current .45.

If you are inclined to experiment,
the .46 button is easy to modify, and a small hobby lathe can
handle the job. Simply machine off enough material from the skirt
of the button so it will fit into the .45 liner. Only one caution
here, the tighter the fit the better the performance. You don't
want a lot of
clearance between the button and liner, just a light press fit, so take off a little
at a time and test fit.